1. Field of the Invention
The present invention relates generally to the field of electrical inspection of electronic devices and in particular to inspection of Liquid Crystal (LC) and Organic Light Emitting Diode (OLED) displays, the systems used in the inspection and defect detection and techniques for image processing in such systems.
2. Description of the Related Art
Liquid crystal display (LCD) panels incorporate liquid crystals that exhibit electric-field dependent light modulating properties. They are used most frequently to display images and other information in a variety of devices ranging from fax machines, cell phones, tablet and laptop computer screens, to large screen, high-definition TVs. Active matrix LCD panels are complex layered structures consisting of several functional layers: one or more layers of polarizing film; a TFT glass substrate incorporating thin-film transistors, storage capacitors, pixel electrodes and interconnect wiring a color filter glass substrate incorporating a black matrix and a color filter array and a transparent common electrode; an orientation film made of polyimide; and the actual liquid crystal material incorporating plastic/glass spacers to maintain proper LCD cell thickness.
LCD panels are manufactured under highly controlled conditions in a clean room environment to maximize yield. Nonetheless, some LCDs may have to be discarded because of manufacturing flaws in the assembled product.
In order to improve LCD panel production yield, multiple inspection and repair steps are implemented during the entire manufacturing process of LCD panels. Among those, one of the most critical inspection steps is array testing, the electrical inspection step performed at the end of the TFT array fabrication process.
There are several conventional array testing technologies currently available to LCD manufacturers in the market, the most prevalent of them being electrical LCD inspection using electro-optical transducers (modulators) as described, for example, in U.S. Pat. No. 4,983,911, incorporated herein by reference in its entirety. One exemplary LCD inspection device of this type is the Array Checker, commercially available from Photon Dynamics, Inc. an Orbotech company located in San Jose, Calif., USA. In particular, the aforesaid Array Checker inspection system employs a method called “VOLTAGE IMAGING®”, which utilizes a reflective liquid-crystal-based modulator configured to measure voltages on individual TFT array pixels. At the time of the inspection of the TFT array by the Array Checker the driving voltage patterns are applied to the TFT panels under test, and the resulting panel pixel voltages are measured by positioning the aforesaid electro-optical modulator in close proximity (typically around 50 microns) to the TFT array under test, and subjecting it to a high voltage square wave voltage pattern. For example, the amplitude of the voltage square wave pattern applied to the modulator could be 300V and frequency of 60 Hz. The electrical potential that is formed across the electro-optical modulator of the inspection system by virtue of its proximity of the pixels of the TFT array under test with applied driving voltage forces liquid crystals in the modulator to change their electric-field-dependent spatial orientation, locally changing their light transmittance across the modulator. In other words, the light transmittance of the modulator becomes representative of the voltages on array pixels in the proximity thereof. To capture the changed modulator transmittance, the modulator is illuminated with a light pulse and the light reflected by the modulator subjected to the panel voltages is imaged onto a voltage imaging optical subsystem (VIOS) camera, which acquires and digitizes the resulting image. The duration of the aforesaid light pulse could be, for example, 1 millisecond. An exemplary system and method for conversion of raw voltage image to LCD pixel map and detection of defects using the map is described in U.S. Pat. No. 7,212,024, incorporated herein by reference in its entirety.
Current methods of defect detection based on voltage imaging use a so-called voltage map, which is a down-sampled version of the original image produced by the aforesaid electro-optical modulator and the VIOS camera. The downsampling is customarily performed based on the size, position and geometry of the LCD pixels of the panel, and the accuracy of the resulting defect detection suffers significantly from the inaccuracy of optical calibration of the inspection system as well as other errors such as glass rotation error, system stage drift and the like.